1. Field of the Invention
The present invention relates to a method of adjusting an optical axis of an optical waveguide element, and an optical waveguide element. In particular, the present invention relates to a method of adjusting an optical axis of an optical waveguide element, which includes an optical waveguide having a Y-branch waveguide formed on a substrate, and to an optical waveguide element. Further, the present invention relates to a method of adjusting an optical axis of an optical waveguide element, which can be used in the field of optical communication or optical measurement, and has a splitter for splitting an input light beam into a plurality of light beams, and to an optical waveguide element.
2. Description of Related Art
In an optical waveguide element having an optical splitter, it is desirable that the light output intensity from the optical waveguide element be evenly split without depending on the wavelengths of light waves input to the optical waveguide element. However, in an actual optical waveguide element, a high-order mode light or a leaky mode light may be excited due to a manufacturing error, misalignment in the position and mode field shape of an incident fiber and an optical waveguide on an element side, or the like, and the branch ratio of the light output intensity in the branch unit may become out of equalization (one-to-one). If an optical modulator having a Mach-Zehnder type optical waveguide is formed with such a branch structure, the ON/OFF extinction ratio may be deteriorated.
In the connectional optical waveguide elements, usually, the waveguide length is increased in order to equalize the branch ratio of light to the branch. The increase in the waveguide length enables the light waves, such as high-order mode light or the like, which has an effect on the branch ratio, to be emitted outside the waveguide before reaching the splitter. As shown in FIG. 7, if the waveguide length (propagation distance) to the branch is longer than 3 mm, the ON/OFF extinction ratio (solid line) can be kept around −20 dB, and the loss difference (dotted line) after the branch can be kept around −0.1 dB.
In addition, as described in Patent Documents 1 to 3, the shape of the optical waveguide has been studied, and the tests for equalizing the branch ratio of light have been performed.
Patent Document 1 discloses a configuration in which a low equivalent refractive index waveguide is provided in an optical waveguide portion ahead of a Y-branch waveguide. With this low equivalent refractive index waveguide, high-order mode light or leaky mode light is radiated from the optical waveguide into the substrate, and only base mode light is propagated, so adjustment is made such that the branch ratio of the light intensity is equalized.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 4-172308
According to the method described in Patent Document 1, the low equivalent refractive index waveguide is provided ahead of the splitter. For this reason, it is necessary to increase the waveguide length before the splitter, which makes it difficult to reduce the size of the entire optical waveguide element.
Patent Document 2 defines the sectional shape and the length of an optical waveguide or a branch waveguide between an incident waveguide and a splitter, thereby suppressing the wavelength dependency of the branch ratio and achieving equalization.
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-326657
According to the method described in Patent Document 2, if a three-dimensional waveguide, such as PLC (Planar Lightwave Circuit) or the like, is used, the optical waveguide can be processed in a specific sectional shape. However, in the case of a so-called diffusion type waveguide, in which an optical waveguide is formed by thermally diffusing a diffusion material, such as Ti or the like, on a substrate made of lithium niobate, it is difficult to process the optical waveguide in a specific sectional shape.
Patent Document 3 discloses the configuration in which a branched optical waveguide for removing high-order mode light is disposed in an optical waveguide ahead of a splitter.
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-181748
According to the method described in Patent Document 3, a general optical waveguide needs to be added to provide an optical waveguide for removing high-order mode light, which leads to deterioration in the manufacturing yield of the optical waveguide element.
Other than the methods of adjusting the shape of the optical waveguide, the optical axes of the optical waveguide element and the input waveguide means are adjusted (referred to as “alignment”) while measuring the light intensity of a light wave emitted from the branch waveguide, or observing the change in the light intensity of the light wave emitted from the Mach-Zehnder type optical waveguide or the output light intensity characteristic (referred to as “modulation characteristic”) from the optical waveguide with respect to means (for example, in the case of a substrate having an electro-optical effect, voltage) for changing the refractive index applied to the optical waveguide (or the phase of light) as a Lissajous waveform by using an oscilloscope, thereby suppressing a shift in the branch ratio.
However, in the alignment method according to the related art, the alignment is made by a single light source, such as a semiconductor laser or the like, having a specific single wavelength. Accordingly, in the case of a specific wavelength, even if the branch ratio is set suitably, when the wavelength to be used changes or when the state of the optical waveguide element changes due to the change in temperature or the like, a shift in the branch ratio occurs, which leads to deterioration in the manufacturing yield concerning the alignment of the optical waveguide element and the input waveguide means.
Meanwhile, in the method of evaluating the characteristic of the optical waveguide element to be manufactured, it is important to determine whether or not the characteristic of the Y-branch waveguide in the optical waveguide element, especially, the branch ratio is suitable. With the method of evaluating the characteristic of the optical waveguide element, similarly to the above-described optical axis adjustment method, a single light source, such as a semiconductor laser or the like, having a specific single wavelength is used in order to evaluate the characteristic of the Y-branch waveguide. In the case of a specific wavelength, even if the branch ratio is evaluated to be suitable, when the wavelength to be used changes or when the state of the optical waveguide element changes due to the change in temperature or the like, a shift in the branch ratio may occur. For this reason, it is necessary to evaluate the characteristic with respect to an input light wavelength or temperature, and simple evaluation is demanded.